Evidence of Long-Term Pair Bonds in Barrow's Goldeneye (Bucephala islandica)

Abstract

Geese, swans, and ducks that cooperate in raising young maintain long-term pair bonds (Kear 1970, Bolen 1971, Weller 1976, Patterson 1982). In most holarctic ducks the female raises the young alone, new pairs are formed every year on wintering and/ or migration areas, and males follow their philopatric female to breeding areas (Hochbaum 1944, Rowley 1983, and others). Female Barrow's Goldeneye (Bucephala islandica) return to the same breeding area every year and often use the same nesting sites (Palmer 1976). Males accompany their mates from wintering areas, defend territories on the breeding ground, and then leave for unknown molting areas when the female is incubating (Savard 1982). They are not seen again on the breeding ponds until the following spring. I present evidence here indicating that some Barrow's Goldeneye pairs remain intact from year to year in spite of a long separation and that pair reunion occurs on the wintering areas. During a study of the breeding ecology of Barrow's Goldeneye in central British Columbia, I captured 15 adult drakes and 81 adult females and marked them with nasal disks. The return rate of females to the study area was 77% (n = 36) in 1982 and 75% (n = 81) in 1983, indicating a high degree of site fidelity. Similar rates of return for females have been found in other cavity-nesting ducks (Erskine 1961, Dow 1983). The return rate of drakes was 71% (n = 7) in 1982 and 63% (n = 15) in 1983. Two of three pairs marked in 1982 returned intact in 1983; the other did not return. The females of the two returning pairs had raised a brood in 1982 and therefore were separated from their mates for at least 4 months. The existence of longterm pair bonds was confirmed in 1984, when 3 of 7 marked pairs returned intact. Of the remaining pairs, 1 split and only 1 member returned in the other 3. The 2 females that had lost their mates had repaired when resighted, but the 4 males had not. These 4 males returned to the same pond where they had been captured the previous year, and 1 even defended a territory for 2 days. Usually, unpaired males do not defend territories (Savard 1982). One Barrow's Goldeneye drake marked on his territory in 1982 defended the same territory in 1983 and 1984. Similarly, 3 other paired males were resighted on the same territory the following year. Although the females of these males were not marked, it is likely that they retained their previous mates because females apparently select the territory in most territorial waterfowl (Hochbaum 1944, Young 1970, Donaghey 1975). The preceding observations indicate that pairs in Barrow's Goldeneye can remain intact from year to year in spite of a long separation and that unpaired males home to their previous breeding area. Homing of unpaired drakes to breeding areas has been reported in several dabbling ducks (Poston 1974, Blohm 1978) and diving ducks (Bengtson 1972, Alison 1975, Donaghey 1975). Breeding philopatry in unpaired males would increase their chances of finding a mate, or of reuniting with a previous mate. It could also enhance their survival because of their familiarity with the resources of the area. I now consider where and how pairs reunite. Within a week of the arrival of Barrow's Goldeneye on their wintering areas in southern coastal British Columbia in early November, some pairs are already defending territories (Savard unpubl. data). Of 34 territories defended along a 5-km stretch of shoreline in Burrard Inlet near Vancouver, B.C. in February and March 1983, 59% were occupied by mid-December and 85% by late December. This rapid formation of pairs soon after the arrival on the wintering areas, when there is little courtship, suggests that pairs reunite then. In 1983, 55% of the 400 males present in Burrard Inlet on 17 November had arrived by 1 November, compared to only 11% of the 200 females. This earlier arrival of the males supports the contention that most pairs reunite on the wintering ground rather than on fall staging areas. Spurr and Milne (1976) found similar pair formation in the Common Eider (Somateria mollissima) that also involved little courtship. Butterfield (1970) also observed that pairs of Zebra Finch (Poephila guttata) that reunited after separation displayed little courtship. I was fortunate to document the reunion of one pair of Barrow's Goldeneye on the wintering area. A drake marked on his breeding territory in 1982 was resighted near Vancouver, B.C. defending a winter territory at the tip of a small jetty. He was paired with an unmarked female, and they remained in their territory all winter. In April 1983 the male defended the same breeding territory as in 1982. He was paired, presumably with the female with whom he had wintered. We marked her that summer, and she raised a brood while the male departed for the molting grounds. On 29 October we sighted the male in a small group of goldeneye (37 males, 3 females, 10 immatures) 2 km east of his winter territory. Daily checks indicated that he remained there until 12 November. On 8 November we sighted his mate in a large group of goldeneye (84 males, 39 females, 1 immature) 4 km from the location of the male. This coincided with the first big influx of adult females